Mechanochemical synthesis of alumina-based catalysts enriched with vanadia and lanthana for selective catalytic reduction of nitrogen oxides

Novel alumina-based materials enriched with vanadia and lanthana were successfully synthesized via in situ modification using a mechanochemical method, and were applied in ammonia-induced selective catalytic reduction of nitrogen oxides (SCR process). The synthesis was optimized in terms of the ball milling time (3 or 5 h), vanadium content (0.5, 1 or 2 wt% in the final product), and lanthanum content (0.5 or 1 wt% in the final product). Vanadium (V) oxide was immobilized on an alumina support to provide catalytic activity, while lanthana was introduced to increase the affinity of nitrogen oxides and create more active adsorption sites. Mechanochemical synthesis successfully produced mesoporous materials with a large specific surface area of 279–337 m2/g and a wide electrokinetic potential range from 60 to (− 40) mV. Catalytic tests showed that the incorporation of vanadia resulted in a very large improvement in catalytic performance compared with pristine alumina, increasing its efficiency from 14 to 63% at 400 °C. The best SCR performance, a 75% nitrogen oxide conversion rate at a temperature of 450 °C, was obtained for alumina enriched with 2 and 0.5 wt% of vanadium and lanthanum, respectively, which may be considered as a promising result.

. To enhance NH 3 adsorption capacity the Al 2 O 3 -vanadia material reported in this study was modified by introducing lanthana during the synthesis. SCR catalysts have previously been modified with La compounds. In the case of formic acid decomposition under SCR-relevant conditions, the addition of a small amount of lanthana to the catalyst led to a base-induced promotional effect 16 . The promotional effect of a basic gas phase reactant (ammonia) on formic acid decomposition activity was achieved catalytically 17,18 . Moreover, lanthana has been reported to cause substantial improvement in the adsorption capability of various materials 19 , which may be beneficial in the case of NH 3 -SCR. This phenomenon is due to the electron configuration in La, which means that its ions can react with functional groups of Lewis acids 20 .
Co-precipitation and sol-gel synthesis are the most popular methods for obtaining oxide systems for use as catalysts in the process of selective reduction of nitrogen oxides [21][22][23] . It is desirable to obtain more environmentally friendly catalysts using a solvent-free synthesis method. Mechanochemical synthesis of advanced materials is currently propagated as a favorable alternative to traditional solution-based methods, which involve heating, addition of expensive or hazardous reagents, and multi-step processing 24 . In conventional chemical synthesis the solvent often plays a key role in energy dispersion, dissolution/solvation, and the transportation of chemicals. An efficient mixing process can overcome the problem of high solvent consumption by enabling solid phase reactions using only nominal amounts of solvent (wet ball milling or liquid-assisted grinding). This approach enables chemical transformations to be induced by mechanical means such as compression, shearing, or friction 25 . In terms of physicochemical properties, mechanochemical synthesis can afford materials with higher surface area and surface energy by altering their structure, chemical composition and/or chemical reactivity throughout the milling process 26 . These parameters are of particular importance in the design and preparation of advanced materials for a range of catalytic applications. There are many reports on the use of mechanochemical protocols to obtain various catalysts (supported metal nanoparticles, nanocomposites, and nanomaterials) with improved catalytic activity and selectivity 24,26,27 , demonstrating the potential of this method to provide more sustainable routes for the preparation of catalysts 28 .
Here we report the application of alumina/vanadia/lanthana hybrids, obtained via mechanochemical synthesis, as catalysts in the selective catalytic reduction of nitrogen oxides with ammonia. The main objectives of the study included: (i) optimization of the mechanochemical soft-templating synthesis of Al 2 O 3 /V 2 O 5 /La 2 O 3 ; (ii) detailed physicochemical and structural analysis of the resulting catalysts; and (iii) practical application of these catalysts in the selective catalytic reduction of NO x . This study is based on the hypothesis that the incorporation of vanadia and lanthana may have a significant impact on the physicochemical and structural properties of the alumina-based materials, as well as on their catalytic performance. It is expected that the combination of alumina as a support, vanadia as an active mass, and lanthana as an activator and element affecting the affinity of the catalyst to the gas phase components should result in the formation of functional materials dedicated for environmental catalysis processes.
Mesoporous oxide materials, based on alumina modified with vanadium species, were obtained by a one-step mechanochemically assisted soft-templating synthesis, similar to that described elsewhere 29 , as shown in Fig. 1.
Approximately 3 g of (EO) 20 (PO) 70 (EO) 20 triblock copolymer (Pluronic ® P-123 from BASF, Co), 1.2 g of alumina precursor (boehmite), an appropriate amount of vanadia precursor (NH 4 VO 3 , ≥ 99.0%, from Fisher Scientific Co.), 5 mL of deionized (DI) water, and 100 μL of HNO 3 (Acros Organics) were introduced to a grinding jar equipped with 8 yttria-stabilized zirconia grinding balls, each of diameter 1 cm. For blank samples, vanadia precursor was not added. In the case of lanthana-modified samples, a specified amount of lanthanum chloride heptahydrate (≥ 99.99%, from Acros Organics) was introduced. The resulting mixture was milled for a set time (3 or 5 h), with a rotation speed of 500 rpm, in a Planetary PM200 mill (Retsch) to pulverize the boehmite particles along with the added metal salts. After milling, the samples were dried at 60 °C for 12 h and then at 100 °C for a further 12 h to evaporate the solvent. The resulting paste-like materials were then calcined in air at 600 °C for 4 h in a tubular quartz furnace to remove the polymeric template and to achieve the desired crystallinity. www.nature.com/scientificreports/ Next, the samples were cooled naturally to room temperature, and without further purification were subjected to physicochemical analysis. The list of prepared samples is presented in Table 1.
Measurements and characterization. X-ray fluorescence (XRF) analysis (Epsilon 4, Malvern Instruments Ltd., UK) and energy-dispersive X-ray spectroscopy (EDX; PTG Prism Si (Li), Princeton Gamma Tech., USA) were used to determine the elemental composition of the obtained materials. Porous structure parameters-BET surface area (S BET ), pore width (w) at the maximum of the pore size distribution, and total pore volume (V t )-were determined from low-temperature (− 196 °C) nitrogen adsorption/desorption isotherms (ASAP 2020, Micromeritics Instrument Co., USA), by the method reported elsewhere 30 . The pore size distribution (PSD) was calculated using the Kruk-Jaroniec-Sayari method calibrated for cylindrical pores and considering the maximum of the curve as the pore width (w) 30,31 . To determine the particle size distribution and particle aggregation, the non-invasive back scattering (NIBS) method was applied, using a Zetasizer Nano ZS (Malvern Instruments Ltd. UK). Scanning electron microscopy (SEM) images were taken using a MIRA3 scanning electron microscope (Tescan, Czech Republic). Powder X-ray diffraction (XRD) measurements were performed on an Empyrean diffractometer (PANalytical, UK) to determine the crystalline structure of the catalyst. Electrokinetic (zeta) potential was determined by the electrophoretic light scattering (ELS) method (Zetasizer Nano ZS, Malvern Instruments Ltd., USA, equipped with an MPT-2 auto titration device).
Catalytic tests. The performance of selected samples was examined in the selective catalytic reduction of nitrogen oxides (NO x ) induced with ammonia. The catalytic process was carried out in a fixed-bed flow microreactor under atmospheric pressure at temperatures from 150 to 450 °C, with 200 mg of catalyst. In a standard run, the reaction mixture (800 ppm of NO, 800 ppm of NH 3 , in He with 3% (v/v) addition of O 2 ) was introduced into the microreactor through mass flow controllers, maintaining a total flow rate of 100 cm 3 min −1 . The catalytic unit downstream of the reactor was used to decompose any NO 2 that might be formed to NO. The concentrations of NO and N 2 O (a by-product of the reaction) in the product stream were measured every 65 s using an NDIR (nondispersive infrared) sensor from Hartmann and Braun. NO conversion was calculated according to the following formula: where NO in is the inlet concentration of NO and NO out is the outlet concentration of NO.

Results and discussion
Synthesis optimization. The focus of this study was the optimization of the synthesis of vanadia-and lanthana-modified alumina and investigation of its catalytic performance in the process of selective catalytic reduction of nitrogen oxides. The first step was to determine the optimal amount of vanadia and grinding time. Increasing the grinding time from 3 to 5 h did not significantly improve the structural properties of the obtained materials, and in some cases the opposite effect was observed (see Table 2). Therefore, on economic grounds, the optimal process time was taken as 3 h. The optimum vanadium content (2 wt%) was determined by gradually increasing the quantity and checking whether the properties of the material were significantly changed. Ultimately, the A-3V2 sample with grinding time 3 h and the highest vanadium content was identified as optimal and was used for further investigations. The conditions for obtaining this sample were further modified by introducing La precursor in quantities equivalent to 0.5 and 1 wt% of La.
Morphology of selected samples. Scanning electron microscopy (SEM) was used to determine the morphology, shape, and size of individual grains of the obtained materials. The SEM images are presented in Fig. 2.
In the case of sample A-3, some large agglomerates are visible. The SEM image also reveals the relatively smooth surface of the particles. The addition of vanadia precursor in sample A-3V2 caused a significant reduction in the size of agglomerates. An increase in the roughness of the material surface can also be observed. In both cases in which Al 2 O 3 was modified with vanadia and lanthana (A-3V2La0.5 and A-3V2La1), the particles consist of    Chemical composition of prepared materials. Determination of the chemical composition of the samples enabled indirect confirmation of the effectiveness of the mechanochemical synthesis of materials based on aluminum, vanadium, and lanthanum oxides. Detailed oxide compositions of all synthesized samples are given in Table 2, and EDX surface mappings of samples A-3, A-3V2, A-3V2La0.5 and A-3V2La1 are shown in Fig. 3. The surface mapping images reveal the uniform distribution of all elements on the samples' surfaces. Furthermore, distinct signals from both doping elements are clearly visible, which provides indirect confirmation of the effectiveness of the synthesis process. An increase in the percentage of vanadium leads to more intense vanadium signals, confirming the higher V 2 O 5 content in the samples, as presented in Table 2. What is more, an increase in the content of vanadium, consistent with the assumptions of the synthesis process (see Table 1), confirms the effectiveness of the proposed modification method.
The XRF spectra obtained for samples A-3, A-3V2, A-3V2La0.5 and A-3V2La1 are shown in Fig. 4a. For all samples, signals originating from aluminum were detected at around 1.486 keV (KαAl). In samples synthesized with ammonium metavanadate, peaks characteristic of vanadium were observed at around 4.949 keV (KαV) and 5.426 keV (KβV); these reflect the vanadium content, which ranges from 1.1 to 4.3 wt%. In the case of samples Porous structure parameters. Low-temperature nitrogen adsorption analysis was used to determine the porous properties of the synthesized materials: surface area (S BET ), total pore volume (V t ), and mesopore size (w) at the maximum of the pore size distributions. Data for all samples are given in Table 2. All samples have a relatively large specific surface area, close to 300 m 2 /g; the lowest value (279 m 2 /g) was obtained for sample A-5 V1, and the highest (337 m 2 /g) for sample A-5V0.5. The total pore volume was not significantly affected by the changes in material's composition. The data also show that increasing the process time from 3 to 5 h did not lead to a significant increase in the surface area of the resulting materials. For this reason, sample A-3V2 was selected as a base material for modification with lanthana. That modification did not significantly affect the surface area of the material. The nitrogen adsorption-desorption curves for samples A-3, A-3V2, A-3V2La0.5 and A-3V2La1 are presented in Fig. 4b. In all curves, the presence of a hysteresis loop is visible. These curves can be characterized as type IV isotherms according to the IUPAC classification, characteristic for mesoporous materials 32 . Isotherms of this type, in combination with an increase in adsorption at high pressures approaching the saturation vapor pressure, are mainly observed for hierarchical porous materials with a wide range of pore size distribution, including meso-and macropores 33 . Moreover, it is known that type IV isotherms represent mono-and multilayer adsorption at low and moderate relative pressures followed by capillary condensation at higher relative pressures 34 . This leads to the conclusion that mechanochemical synthesis is an effective method for obtaining mesoporous alumina/vanadia/lanthana materials.
Crystalline structure of the obtained materials. The XRD patterns of the obtained powders are presented in Fig. 4c. The patterns for all samples contain diffraction peaks corresponding to signals originating from Al 2 O 3 with cubic crystal symmetry exhibiting a face-centered lattice. Signals originating from α-Al 2 O 3 (peaks at 400, 422 and 620), γ-Al 2 O 3 (peaks at 220, 311, 222 and 140) and θ-Al 2 O 3 (peak at 140) are visible in the patterns. All diffraction peaks in the patterns suggest that they are consistent with the standard Al 2 O 3 pattern (JCPDS database, Card Number 79-1558) [35][36][37] . The addition of V or La had no significant impact on the course of the curve, and thus on the crystalline structure of the sample. No diffraction peaks from any other chemical species are detectable in the diffraction patterns.
Surface charge properties. The materials surface electrokinetic potential may be important for assessing the effectiveness of adsorption processes, being a determining factor for catalytic degradation efficiency. The value of the surface charge of the material controls its bonding with the adsorbate via the mechanism of electrostatic attraction. Figure 4d shows the electrokinetic potential as a function of the pH of the solution for the four selected materials. For all tested materials, the curves follow the same trend-the materials have positive zeta potential at slightly acidic pH, reaching the isoelectric point (IEP) around a neutral pH of 6-8, and developing a negative charge in a more basic environment. The zeta potential values decrease with increasing pH due to the smaller number of H + ions. In more alkaline environment, due to the presence of hydroxyl ions (OH − ), the negative charge builds up at the sample surface 38 . Consequently, an increase in pH may lead to a reduction in the zeta potential value. For the pristine Al 2 O 3 material (A-3) the maximum zeta potential is 43 mV and the minimum is − 21 mV. The addition of vanadia precursor during the synthesis caused a reduction in the material's potential range, to 33-(− 39) eV. Similar changes in the potential have been observed in the case of other inorganic oxides doped with vanadium 39 . While the addition of vanadia precursor reduces this range, the incorporation of lanthana results in a significant increase in the pH range in which the zeta potential remains positive. The presence of a positive charge on the material's surface is probably related to the protonization of OH groups. www.nature.com/scientificreports/ The isoelectric point (IEP), evaluated from the graph of zeta potential vs. pH, plays an important role in the process of heterogeneous catalysis, because it affects the adsorption capacity of reactants on the catalyst surface 40 . The IEPs of A-3, A-3V2, A-3V2La0.5 and A-3V2La1 were found to lie at pH values of 5.6, 4.4, 7.3 and 7.9, respectively. This means that the materials obtained are positively charged in solutions with pH values lower than these points, and negatively charged in solutions with pH above these points.
Catalytic activity. The catalytic abilities of selected aluminum oxide-based materials were tested in the process of ammonia-induced selective catalytic reduction of NO x , within the temperature range 150-450 °C. The test results are presented in Fig. 5.
Generally, as the reaction temperature increases, an upward trend in the reduction of nitrogen oxides is observed, as expected. The lowest catalytic activity is observed for pure Al 2 O 3 (A-3), which achieves a maximum reduction of 14% at 400 °C. For this material, the beginning of NO x production is observed above 400 °C. A very large improvement in catalytic activity is observed for the vanadium-containing sample A-3V2 in comparison with the pristine material. The highest NO x reduction rate of 63% was obtained with this material at 400 °C, and was significantly higher than the result for the pure Al 2 O 3 material. This is probably related to the fact that the enrichment of alumina with vanadia significantly improves its catalytic properties. As mentioned in the introduction, ammonia is strongly adsorbed adjacent to V = O sites as NH 4 + , and the reaction rate is directly proportional to the number of surface V = O bonds 15 , which facilitate the SCR reaction. Moreover, the presence of nitrogen in the form of NH 4 + may significantly strengthen the catalytic activity of the materials. Unfortunately, for sample A-3V2, the conversion of NO x at temperatures above 400 °C was found to be problematic. However, for the samples modified with lanthana this problem disappeared. The incorporation of lanthana into the structure of the Al 2 O 3 /V 2 O 5 materials did not significantly improve their catalytic performance in the temperature range 150-400 °C, but enabled elimination of the production of nitrogen oxides at temperatures above this range. This behavior may also be due to the wider range of positive surface charge of lanthana-modified samples. Byproduct production was constantly monitored during the experiments, and the results are presented in Fig. 5b. For all samples, the amount of by-product increased gradually with increasing reaction temperature. However, in the case of vanadia-and lanthana-modified samples the amount of N 2 O did not exceed 23 ppm in the whole temperature range. For the pure Al 2 O 3 sample higher N 2 O production was observed, reaching almost 31 ppm at 450 °C, which is nevertheless fairly low. Based on the results, a mechanism for the catalytic action of Al 2 O 3 / V 2 O 5 /La 2 O 3 materials was proposed (see Fig. 6).
Nitrogen oxides are adsorbed on the surface of Al 2 O 3 /V 2 O 5 /La 2 O 3 and, due to the presence of active vanadium sites, ammonia reacts with them. Moreover, the incorporation of lanthana causes an increase in the number of active sites on the catalyst's surface, which facilitates the adsorption of nitrogen oxides and thus makes their selective catalytic reduction more effective. Ammonia holds a dominant position in competitive adsorption between NH 3 and NO on similar vanadium-containing metal oxide catalysts, and so analogous behavior probably occurs in the case of Al 2 O 3 /V 2 O 5 /La 2 O 3 . Therefore, the bonds between nitrogen and oxygen are broken, enabling the formation of molecular nitrogen, water, and N 2 O. Nitrogen oxides can be adsorbed on the surface of the catalyst and then transformed into monodentate species of nitrite and nitrates, which is favored by the presence of O 2 and significantly inhibits NH 3 adsorption, hindering the SCR reaction 41 .
Comparison with previous studies. Alumina-based materials modified with various metal species have previously been used as catalysts in SCR processes. However, they have mostly been investigated in hydrocar-   O 3 achieved NO x conversion rates of 0%, < 40% and < 60%, respectively. By contrast, synthesized under presented study A-3V2La0.5 material enabled to achieve a 70% conversion rate at 400 °C, higher than that achieved by the Pt-Mo/Al 2 O 3 -La 2 O 3 sample, justifying the claim that vanadium compounds are promising modifiers of catalysts for SCR processes.

Conclusions
In this work, an effective mechanochemical synthesis of alumina-based oxide materials, and their successful enrichment with vanadium and lanthanum species were accomplished via in situ modification. The presence of V (1.1-4.3 wt%) and La (0.5-1.1 wt%) in the modified samples was confirmed by EDX analysis. XRF analysis further confirmed the presence of these elements in the oxide forms V 2 O 5 (2.2-8.8 wt%) and La 2 O 3 (1.1-2.0 wt%). All nitrogen adsorption-desorption isotherms were classified as type IV, characteristic for mesoporous materials, due to the presence of a hysteresis loop. The proposed synthesis route led to materials with a large specific surface area of 279-337 m 2 /g. While SEM images of pristine Al 2 O 3 show agglomerates of significant size, all other samples reveal irregular structures with various grain sizes and degrees of agglomeration, which suggests that the introduction of additional metal-containing species to Al 2 O 3 increases the structural heterogeneity of the material. Surface charge properties were determined by measurements of electrokinetic potential. All of the samples exhibited similar curves, with positive zeta potential values at acidic pH, reaching the isoelectric point around a neutral pH of 6-8, and developing a negative charge in more basic environments. The incorporation of vanadia caused a decrease in the zeta potential, while the addition of lanthana caused its significant increase. It is concluded that the proposed mechanochemical synthesis is an effective method for obtaining mesoporous alumina/vanadia/lanthana hybrids.